Based on the Chinese patent “Capacitive Fingerprint Sensor” with the application number of 201210403271.2, in the technical solution of the capacitive fingerprint sensor provided by the Chinese patent “Capacitive Fingerprint Sensor” with the application number of 201410004072.3, a sensing equation and a circuit model are optimized to provide a novel circuit structure so as to realize “C-Q-T” conversion.
The capacitive fingerprint sensor images fingerprints by measuring differences between the coupling capacitance formed between ridge lines of fingerprints and planar sensing electrode array units and that formed between valley lines of the fingerprints and the planar sensing electrode array units. When a dielectric layer between a sensing electrode array and a finger becomes thicker, the capacitance may decay and images of the sensor will be increasingly blurred. With technological development, the thickness of a dielectric layer between a sensing electrode and a target electrode of a commercial capacitive fingerprint sensor increases from an order of magnitude of 10 μm to an order of magnitude of 100 μm. Furthermore, taking into account the integrity of industrial design of a mobile phone screen, it is expected that the fingerprint sensor can directly penetrate through screen glass, particularly chemically strengthened glass with the thickness of 400 μm to 500 μm. So, improving the imaging capability of the fingerprint sensor as much as possible becomes an important indicator for measuring the performance of the fingerprint sensor at present.
To improve imaging of a “C-Q-T” type fingerprint sensor, a driving signal can be coupled between the finger and the fingerprint sensor, and the amplitude of the driving signal can be increased to enhance a measured signal to further enhance fingerprint imaging. One of two common driving modes is to directly couple the finger with the driving signal, and the other is to couple a power supply end or a ground end of the fingerprint sensor with the driving signal. The two driving modes can equivalently enhance the fingerprint imaging capability when using theoretical models. However, in practical application, it is difficult to model accurately due to relatively complicated electrical characteristics of the finger. Besides, different individuals have different electrical characteristics under different temperatures and humidity, so that the design of the method for directly coupling the finger with driving is inconvenient. On one hand, the frequency of the driving signal for driving the finger should not be too high, since the driving signal with the relatively high frequency may lead to an amplitude gradient when transmitted on the surface of the finger to result in inconsistent amplitude of the driving signal on the surface of the finger, which probably causes field inconsistency in fingerprint imaging. On the other hand, the amplitude of the driving signal applied to the finger should not be too high, either. If the peak-peak value of the driving signal exceeds 4 V, discomfort may be caused to a human body.